Hello everyone, I thought of this problem yesterday: Given a matrix of length N and width M, answer Q queries of this form:
- x₁ y₁ x₂ y₂: Count the number of distinct elements in the submatrix with corners (x₁, y₁) and (x₂, y₂).
There is a version of this problem on Codechef, however the bounds for A_i, j are very small. Is there a way to solve this for e.g.

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UPD: It turns out that this problem is named "colored orthogonal range counting". It has been solved in "Further Results on Generalized Intersection Searching Problems: Counting, Reporting, and Dynamization" and "Efficient Colored Orthogonal Range Counting".